In an information recording medium, such as an optical disc, like a CD-ROM (Compact Disc-Read Only Memory), a CD-R (Compact Disc-Recordable) or the like, there is also developed a multilayer type optical disc or the like on which a plurality of recording layers are laminated on a same substrate. More specifically, a dual-layer type optical disc, which is one specific example of the multilayer type optical disc, has a first recording layer (referred to as a “L0 layer” as occasion demands in the application) located on the front side (i.e. on the closer side to an optical pickup) viewed from the irradiation side of laser light in the recording by an information recording apparatus, and has a semi-transmissive reflective film located on the rear thereof (i.e. on the farther side from the optical pickup). The dual-layer type optical disc also has a second recording layer (referred to as a “L1 layer” as occasion demands in the application) located on the rear side of the semi-transmissive reflective film through an intermediate layer, such as an adhesive layer, and has a reflective film located on the rear side thereof. In preparing such a dual-layer type information recording medium, the L0 layer and the L1 layer are formed separately, and the two layers are bonded in the end, by which the dual-layer type optical disc can be manufactured at low cost.
Then, an information recording apparatus, such as a CD recorder, which performs recording with respect to the dual layer type optical disc, focuses (or irradiates) laser light for recording on the L0 layer, to thereby record data into the L0 layer in an irreversible change recording method or in a rewritable method, and focuses the laser light on the L1 layer, to thereby record data into the L1 layer in the irreversible change recording method or in the rewritable method.
On the other hand, in reproducing the data recorded on the optical disc, in order to maintain the preferable data reproduction, it is necessary to eliminate a direct current potential difference (or DC level change) which is superimposed on a detection signal (i.e. RF signal) obtained by that the laser light irradiated from the optical pickup is reflected on a recording surface of the optical disc. Thus, a measure of binarizing the detection signal after eliminating the direct current potential difference of the detection signal with a high pass filter (refer to patent documents 1, 2, and 3), and a measure of binarizing after adding an offset to the slider level of a data slicer (refer to a patent document 4), and the like are developed. In these measures, the extent of the offset added to the slider level of the data slicer and the time constant (or cutoff frequency) of the high pass filter are determined depending on a defect caused on the surface of the optical disc by a scratch, fingerprints, or the like, or depending on a difference in recording features between a data unit and a header unit accompanying the data unit.    Patent document 1: Japanese Patent Application Laid Open NO. 2003-46373    Patent document 2: Japanese Patent Application Laid Open NO. 2002-123945    Patent document 3: Japanese Patent Application Laid Open NO. 2000-182239    Patent document 4: Japanese Patent Application Laid Open NO. 2002-319239